usb: usb251xb: fix to avoid potential NULL pointer dereference

of_match_device in usb251xb_probe can fail and returns a NULL pointer.
The patch avoids a potential NULL pointer dereference in this scenario.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Reviewed-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>